No cell better represents the evolutionary tussle between males and females, though more often it's a tussle between males for females.

In the face of competition from other males, animals, from fish to foul, have evolved larger, faster and more streamlined sperm cells, which are among the most diverse of all animal cells.

"The variety is even more puzzling given the fact that they all have the same goal," Simone Immler of the University of Sheffield.

One trait that makes sperm faster - and therefore more likely to beat another male's cells to the egg - is size. Whip-like tails called flagellum propel sperm through fluids, and the longer the better. To drive those oversized flagella, cells grow larger mid-pieces - essentially the engine of a sperm cell - to hold extra mitochondria.

Immler found that these characteristics can change in an individual animal, depending on the competition it faces.

Male Gouldian finches come in two varieties: dominant redheads and subordinate blackheads. Redheads get first dibs on females and food, and Immler wanted to know how this social structure affects the size of their sperm. She and her colleagues paired three birds together, in various combinations of redheads and blackheads, with females in sight, but out of reach.

After four weeks, the result was clear: competition breeds longer sperm cells. In most of the groups the flagellum grew slightly longer, and sperm cells from both kinds of males tended to grow longer mid-pieces.

But one problem with super-sized sperm cells is that they take up more room in the testes, explained Stefan Leupold, another Sheffield University sperm expert. Animals can compensate by growing larger testes, but the things can only get so big.

To figure out how animals solve this quandary, Leupold studied 21 species of blackbirds and cowbirds, extremely promiscuous birds with huge variation in sperm size.

Birds that make larger sperm cells have big semenifurous tubules - the structure in the testes that gives birth to sperm - and they pack them densely into their testicles, Leopold and colleagues found.

But not all big sperm cells are necessarily fast, said John Fitzpatrick, of McMaster University in Hamilton, Ontario. Among cichlids - popular and colourful aquarium fish native to African lakes and streams - males appear to have evolved powerful and fast sperm cells, which later evolved to be large, Fitzpatrick said.

"You can increase the size of the car all you want, but if you don't have the engine to propel it, you're not going to go anywhere," he said.Ewen Callaway, online reporter